This invention relates to the art of environmental control systems, to thermostats or thermostat apparatus which control heating, ventilation and cooling systems, and in particular to those thermostats which control a heating system that includes a heat pump apparatus in addition to apparatus that provides for additional or alternative auxiliary heating. Such apparatus includes a heat pump or “compressor” which incorporates also some form of resistive heating, and also heat pumps used in conjunction with heat strips or resistive heating apparatus which are not incorporated in the actual cabinetry of the heat pump. More generally, the present invention relates to heating systems and the control of heating systems which incorporate multiple means of heating, each with its own level of efficiency in energy usage.
Energy usage for environmental control, especially for heating or cooling, is a major expense in the entertainment and motel/hotel industries. Even minor changes in environmental control procedures can result in significant energy and cost savings. Typical approaches for saving energy include such procedures as manually turning off a heating or cooling system when a room is not used, or reducing the level of heating or cooling based upon some sequence of programmed control by a thermostat.
Energy savings can be achieved by use of a control mechanism which takes into account whether a room is predicted to be occupied, and further savings can be achieved if the detection of occupancy and a programmed response to changes in occupancy is automated. Therefore, in order to provide a thermostat with information regarding occupancy of a room or area, designers of thermostats have incorporated motion detectors or occupancy sensors, or have provided for input to the thermostat from an external occupancy sensor which informs the controlling mechanism of the thermostat when the conditioned space served by space conditioning equipment (a heating, cooling, and/or ventilation system) is “unoccupied” meaning that no people (or animals) are detected, presumably in the conditioned space, by the occupancy sensor.
Occupancy sensors are implemented using various types of motion detectors, such as infrared motion detectors, and could be implemented with other mechanisms which sense the presence of a person in a room, or which sense entry of a person into a room or conditioned space. These and other methods are known in the art or could be devised by one knowledgeable in the state of the art.
In the entertainment or hotel/motel industries, considerable energy savings can be gained, during periods when the outside temperature is low and heating is normally required, by reducing the temperature setpoint when a room is unoccupied. During a time when heat is typically needed, and a specific room or space is unoccupied, energy can be saved by reducing the temperature setpoint as utilized by the thermostat in that room from the “normal” setpoint, that is, the normal occupied setting. For example, if the room temperature normally desired when a room is occupied was 72 degrees Fahrenheit, and the outside temperature was 40 degrees, it would save energy to set the setpoint temperature to 62 degrees when the room is unoccupied, and then return the setpoint temperature to 72 degrees anytime occupancy is again detected. More savings in energy costs could result if the setpoint temperature was reduced even further to less than 62 degrees. Reducing the setpoint temperature by one degree Fahrenheit can save several percent on energy costs, and even eliminate the need for any heat on days when outside temperatures are not too much lower than a reduced setpoint temperature. One government study by the U.S. Department of Energy showed about a five percent savings for one degree Fahrenheit reduction in setpoint temperature over an entire year (1997 study from http://www.eia.doe.gov/emeu/consumptionbriefs/recs/thermostat_settings/thermostat.html).
However, reducing the temperature of a room during periods of no occupancy can have the unwanted side effect that the room may be uncomfortably cold when a guest returns after a period of non-occupancy, and if the heater cannot respond to warm the room quickly enough this may make the guest feel cold, unhappy, and/or dissatisfied.
One of the more efficient devices for heating with regards to energy usage is a heat pump. Within a range of outside temperature, a heat pump can produce more energy in the form of heat from a kilowatt of input energy than can a heat strip or electric coil heater. Using a heat pump instead of electric heat strips to heat a room can reduce energy usage by a factor of one-half or better. That is, one kilowatt of input power applied to a heat pump can produce anywhere from one to three kilowatts of equivalent heating energy in comparison to use of electric heat strips, radiant heat, or any form of resistive heating. A heat pump is most efficient when outside temperatures are near to a desired indoor temperature. For example, if a desired indoor temperature is 72 degrees Fahrenheit, then a heat pump would be efficient at temperatures near 72 degrees. Heat pump efficiency decreases at lower outside temperatures, and at temperatures somewhere around freezing, a heat pump is no longer more efficient than resistive heating. The characteristics of such heat pump technology are well known in the art.
Heat strips, radiant coils, or other similar methods of producing heat by converting electrical energy directly to heat energy are commonly called “resistive” heating. “Resistive” meaning that heat is produced by running electricity through a resistive medium and thus producing heat. Heat strips can be easily manufactured at low cost, and a typical heat coil or heat strip can typically produce more heat in a short time than what can be achieved by a heat pump, but typically in a less energy efficient manner. That is, a heat strip or resistive heating mechanism can typically produce more heat than a heat pump or can produce heat in addition to a heat pump, but at higher cost than use of a heat pump alone.
Therefore, heat pumps are typically used for both residential and commercial heating for economical reasons and resistive heating is typically used only when outside temperatures are such that heat pumps either no longer can produce the required heat, or at which efficiency of the heat pump is less than that of resistive heating. It is noted also that heat pumps are typically implemented as apparatus which can either cool or heat, that is, a heat pump can “pump” heat either into or out of a room or building. Resistive heating however cannot be used directly for cooling, that is, there is no way to run a resistive heating coil or strip in “reverse”.
It is further noted that although heat pumps producing “compressor” heat will often be more efficient in their use of energy than heat strips or resistive heating, that there are times when resistive heating is required to supplement or replace use of the heat pump. The choice of whether resistive heat is used by itself when necessary or as a supplement to heat pump heating depends on several factors such as local building codes, the size of the wires feeding the heat pump/resistive heating strips, the size of the breaker or fuse feeding the heat pump unit, or other factors. These considerations are well known in the art.
In a thermostat apparatus for control of a heating apparatus providing heat to a room or conditioned space in which the thermostat apparatus includes occupancy or non-occupancy in determination of a setpoint temperature, energy can be saved by reducing the setpoint temperature of the room when the room is not occupied. When the room then becomes occupied after a period of no occupancy, the “recovery time” in restoring the room temperature to its desired value is important in keeping the occupants of the room warm and happy. People may not tolerate having the room too cold for any extended length of time after they occupy the room. During a period of no occupancy, the highest level of energy savings would be achieved if the heating apparatus serving the conditioned space was completely disabled, or maybe a safer alternate approach would be to set the setpoint temperature very low such as just above freezing (32 degrees Fahrenheit) to keep the water pipes or any other water in the room or fixtures of the room from freezing. This would mean that whenever the conditioned space was unoccupied, the heater would be turned off, and the temperature in the conditioned space would drift lower towards the outside temperature. Then, when someone entered the room the heater would be turned on and it would take some period of time before the heater could heat the conditioned space back to the desired temperature. This time for returning the temperature in the room to the user's or occupant's desired room temperature, following a period of non-occupancy during which temperature in the room has been reduced is termed the “recovery time”.
Since many outside factors come into play in determining how fast the temperature in a room will change when heat is applied, it would not provide maximum benefit for the thermostat to simply use some predetermined fixed rate of room response in calculation of a non-occupied setpoint temperature. It is also not of greatest benefit to simply use a predetermined setback which does not take into account current conditions. Approaches such as these, if properly programmed, may provide for acceptable recovery times but would not provide for maximized energy savings, or if programmed for good energy savings the recovery time would be likely go beyond what would be tolerated by the occupants.